Composite articles incorporating honeycomb cores are commonly utilized for fabricating aerospace structures due to their advantageous strength to weight ratio. Honeycomb core (HC) composite articles are typically comprised of upper and lower composite skins, i.e., fiber reinforced resin matrix laminates, that are separated and stabilized by the honeycomb core. Due to the high bending stiffness and compressive strength properties of HC composite articles, i.e., the honeycomb core functions as a shear web and spaces the composite skins from the bending neutral axis, HC composite articles have particular utility in aerospace applications such as aircraft fuselage panels and door structures. The high strength and low weight of such construction results in lower overall aircraft system weight.
HC composite articles may be fabricated utilizing various composite forming methods. The most commonly employed technique involves the use of a vacuum bag molding assembly wherein an impervious membrane or "vacuum bag" is employed for consolidating the composite skins and ensuring proper adhesion thereof to the centrally disposed honeycomb core. More specifically, the lower composite skin, honeycomb core, and upper composite skin are sequentially laid in a rigid mold member so that the honeycomb core is overlaid by the upper and lower composite skins. The upper and lower composite skins are formed from uncured "prepreg" or "B-stage" laminates comprised of a fiber reinforcement such as graphite, aramide or fiberglass fibers disposed in a binding matrix such as epoxy, phenolic or other similar organic resinous material. Film adhesive, which is applied to the honeycomb core prior to the lay-up, forms the bonds between the upper and lower composite laminates and the honeycomb core. The vacuum bag is disposed over the rigid mold member and sealed thereto so as to form a mold cavity which is occupied by the uncured composite lay-up. The mold cavity is then evacuated and additional pressure and temperature are applied via an autoclave oven to cure the lay-up. The combination of vacuum and external pressure functions to consolidate the composite skins, remove air and volatiles from the resin binder, and apply the necessary compaction pressure to ensure full and uniform adhesion of the lay-up.
Difficulties commonly encountered during the fabrication of HC composite articles relate to shifting and/or distortion of the honeycomb core under compaction pressure. While the honeycomb core is relatively stable in the direction of the individual cells, i.e., the cells provide significant buckling stability, it will be appreciated that pressure applied transversely of the cells may cause distortion and/or shifting, e.g., accordioning, of the honeycomb core due to the inadequate strength thereof in a lateral direction. This is more clearly understood by reference to FIG. 1a wherein a lay-up of upper and lower composite skins 100, 102, and a honeycomb core 104 is disposed in a vacuum bag molding assembly 108. The vacuum bag 110 is shown applying a lateral component of pressure P along the ramped edges of the honeycomb sore, which lateral pressure component causes the local collapse and distortion of the honeycomb core edges. FIG. 1b shows a top view of the cured HC composite article wherein the distortion, indicated by dashed lines 112, is exaggerated for illustrative purposes.
Attempts to overcome problems of distortion and shifting have included stabilization techniques wherein the edges of the honeycomb core, i.e., several rows of honeycomb cells about the entire periphery, are stabilized by the application of film adhesive or filled with a low density syntactic foam. Once cured, the film adhesive and/or the foamfilled cells serve to retard the accordioning of the honeycomb core. U.S. Pat. Nos. 4,680,216 and 5,354,195 discuss honeycomb core stabilization techniques and various materials useful therefor. While these techniques have been marginally successful in limiting distortion of the honeycomb core (on the order of about 0.64 cm to 0.95 cm (0.25 in to 0.375 in), such materials are substantially parasitic and are not practical for applications wherein minimization of overall aircraft weight is a critical design criterion. Furthermore, these stabilization options are not acceptable for applications wherein accurate and distortion-free core location is highly critical. For example, applications requiring the use of radar absorbent (i.e., carbon-loaded) honeycomb core to defeat enemy radar require far more exacting manufacturing tolerances than those which can be produced by prior art stabilization techniques. If shifting of the radar absorbent honeycomb core should occur during the manufacturing process, radar coverage on the aircraft could be compromised.
Other attempts to yield a distortion-free core have included the use of restraint devices formed or assembled about the periphery of the molding assembly. FIG. 2a depicts a vacuum bag molding assembly wherein rows of vertically protruding pins 120 are affixed to a rigid mold member 122 and disposed in adjacent relation to the HC composite article 124 to be formed. As the upper composite skin 126 is laid over the honeycomb core 128, the pins 120 are caused to engage a peripheral portion 130 of the upper composite skin 126, i.e., pierce the composite fabric, to prevent lateral displacement thereof during the molding/compaction process. A bridging effect is thereby created in the upper composite skin 126, i.e., between the uppermost corner 132 of the honeycomb core 128 and the mating surface 134 of the lower composite skin 136, to react lateral compaction pressure and, consequently, prevent distortion of the honeycomb core 128. While this technique is suitable for high tolerance applications, e.g., LO applications, the protruding pins 120 are a source of high maintenance, i.e., requiring periodic cleaning and repair, pose a hazard to the operator, and create difficulties when sealing the vacuum bag 138 to the rigid mold member 122. Regarding the latter, the vacuum bag 138 must be sealed outboard of the protruding pins 120, thus requiring the additional step of disposing a protective elastomer strip 139 over the protruding pins 120 to prevent damage to the vacuum bag 138.
A similar approach is shown in FIG. 2b wherein a perforated or apertured metal strip 140 is substituted for the protruding pins 120. The peripheral portion 130 of the upper composite skin 126 is laid over the apertured metal strip 140 such that under compacting pressure the apertures 142 thereof capture or grip the peripheral portion 130 to prevent lateral displacement of the upper composite skin 126. This approach yields similar results to the above-described pinned configuration, however, laborious cleaning is required to remove excess resin from the apertures 142 prior to initiating the next cure cycle.
A need therefore exists to provide an improved method of manufacturing HC composite articles which provides accurate and distortion-free core location and minimizes repair and/or maintenance of the molding assembly.